Brazing has been known for ages as one of metalworking techniques whereby mating metals can be joined with each other with relative ease. Since brazing is among important metalworking techniques of even today, it is applied to a wide variety of industrial sectors, and kinds of brazing filler metal for use in the brazing cover a broad spectrum.
For some kinds of metals and their alloy, however, there has not been found an effective brazing filler metal as yet. Stainless steel is one of such metals. As stainless steel has excellent properties such as less corrosiveness because of high corrosion resistance and high resistance to acid and heat, it is in widespread use in various industrial sectors, and is used even for metallic articles (for example, a wrist watch, the frame of eyeglasses) of which decorativeness in external appearance is required.
By the way, as described in an article titled “Story of Stainless Steel” compiled by Japan Institute Standards Organization (issued by Japan Institute Standards Organization Press), silver solder and nickel solder have been well known as conventional brazing filler metals for use in brazing of members made of stainless steel (referred to hereinafter merely as “stainless steel members”).
Silver solder as a brazing filler metal has its own melting point in a range of from about 800 to 1000° C. Examples of a brazing filler metal include a brazing filler metal with which brazing can be executed at a temperature not higher than 800° C., for example, BAg-8 (melting point: 780° C.) according to the JIS specification, which is at times put to use in brazing of the stainless steel members. However, such a brazing filler metal as described is not in much use for brazing in fabrication of metallic articles such as a wrist watch, and the frame of eyeglasses, of which decorativeness in external appearance is required, because the brazing filler metal has poor corrosion resistance and is prone to initiation of corrosion of the stainless steel members after brazing.
Examples of nickel solder include BNi-2 (melting point: 1000° C.) according to the JIS specification. Normally, there occurs no coarsening of the crystal structure in a reducing atmosphere such as hydrogen at about not higher than 800° C. In this case, however, since this brazing filler metal has a high melting point which is higher than 800° C., brazing is executed using this brazing filler metal, thereby causing the crystal structure of the brazed stainless steel members to be coarsened. Accordingly, in the case of brazing the stainless steel members with BNi-2, there is the need for taking steps of removing coarsened portions of the crystal structure thereof by grinding, and so forth, and subsequently applying thereto mirror-finish, and the like. It should be noted that members to be brazed which are objects of brazing are called “base metals” in the following description, which mainly denote stainless steel members in the invention. When stainless steel is used as base metals, the “crystal coarsening temperature of the base metals” in this case is set at 800° C.
On the other hand, a brazing filler metal (melting point: 1000 to 1200° C.) containing Cr, Fe, Si, and Ni as main constituents thereof suitable for stainless steel is disclosed in the Japanese publication of examined patent application No. 61-10235, which discloses that brazing was implemented at 1050 to 1250° C. using the brazing filler metal. This brazing filler metal, however, also has a melting point not lower than the crystal coarsening temperature of the base metals, thus causing the crystal structure of the stainless steel members to be coarsened after brazing.
As a method of joining stainless steel members together, welding is in widespread use besides brazing. Welding has no problem in respect of joining strength and corrosion resistance, however, there emerge spots where the stainless steel members need to be heated locally to a high temperature for implementing joining. As a result, there have emerged spots where a temperature is in excess of the crystal coarsening temperature of the base metals, causing the crystal structure thereof to be coarsened, so that welding has a drawback in that portions of the stainless steel members, worked on by welding, require post-working. Also, there are times when projection welding is applied for joining of stainless steel members whereby welding is executed by causing electric current to flow through projections of the stainless steel members, formed for the purpose of welding. The projection welding, however, has had a drawback in that, if the stainless steel members to be joined with each other are complex in construction, it becomes difficult to cause uniform concentration of electric current on the projections of the stainless steel members, thus resulting in difficulty with welding.
As described in the foregoing, conventional techniques for joining the stainless steel members have not offered any joining techniques capable of securing excellent corrosion resistance and sufficient joining strength, and further, capable of implementing joining at a temperature which does not cause the crystal structure thereof to be coarsened.
The invention has been developed to solve the problems as described above, and it is an object of the invention to provide a brazing filler metal for use in brazing of metals such as stainless steel, used in members of which decorativeness in external appearance is required, wherein brazing can be implemented at a temperature which does not cause the crystal structure thereof to be coarsened while securing excellent corrosion resistance and sufficient joining strength.